In recent years, several individual location determination systems have been proposed and applied to provide estimates of varying accuracy for location of an object in special circumstances. For an object located outdoors and away from obstructions such as groups of buildings, a satellite-based LD system such as the Global Positioning System (GPS) or the Global Orbiting Navigation Satellite System (GLONASS) or other similar system can be used to provide location estimates with associated inaccuracies as low as ten meters or less. A ground-based system, such as Loran, Omega, Tacan, Decca, JTIDS Relnav or PLRS, can provide location estimates with associated inaccuracies as low as 60 meters or less in the same situations, using triangulation based on intersections of hyperbolic surfaces. These systems provide object location estimates over regions with diameters of hundreds of kilometers (km) but do not work well where some of the signal sources are obscured by structures outdoors, or when the object to be located is positioned indoors.
Some cellular telephone-based systems also provide estimates of location, using comparison of signal strengths from three or more sources. FM subcarrier signals can be used over smaller regions to estimate the location of an object inside or outside a building or other structure, even where the signal sources have no line-of-sight to the object. Use of cellular-based systems and FM subcarrier systems for location determination tends to be limited to smaller regions,. with diameters of the order of 20-50 km.
FM subcarrier signals and AM carrier signals have been used for some types of radio wave communications. In U.S. Pat. No. 3,889,264, Fletcher discloses a vehicle location system in which the unsynchronized AM carrier signals from three or more AM radio stations form hyperbolic isophase grid lines that are used to determine location of a vehicle. The vehicle must be equipped with a three-channel, tunable receiver, and its location must be referenced to an initial known location by counting the number of isophase lines crossed after the vehicle leaves the initial location. Isophase drift is compensated for by subtraction from the count.
Dalabakis et al, in U.S. Pat. No. 4,054,880, disclose a radio navigation and vehicle location system employing three low frequency subcarrier signals received from three radio stations at a three-channel, tunable receiver located on the vehicle. Isophase lines crossed are counted after the vehicle leaves an initial known location. This system, like the Fletcher system, is a delta-position system that determines vehicle location only relative to an initially known location.
U.S. Pat. No. 4,646,290, issued to Hills, discloses use of F.C.C.-approved Subsidiary Communication Authorization (SCA) FM subcarrier signals for one way transmission. This patent discloses transmission of a plurality of messages, which may be delivered to the transmitter at a wide range of bit rates, to be transmitted at a single bit rate that is at least as large as the highest bit rate for message delivery. This method allows for downstream insertion of additional data.
An integrated radio location and communication system for a mobile station is disclosed by Martinez in U.S. Pat. No. 4,651,156. Each mobile station carries a transceiver that issues radio signals that are received by two or more signal transceiver reference sites having fixed, known locations. The transceivers at the mobile station and the reference stations are continuously phase locked to the RF carrier signal from a nearby commercial radio station. The radio station and the mobile station each transmit a brief, distinguishable range tone at a known sequence of times, and the range tone from each station is received by each reference station. From an analysis of the differences in arrival times of the range tones received from the radio station and from the mobile station, the reference stations determine the two-dimensional location of the mobile station. The mobile station uses the beat signal between two RF subcarrier frequencies to generate its range tone signal and to distinguish that mobile station transmissions from the transmissions of any other mobile station.
Young et al, in U.S. Pat. No. 4,660,193, disclose use of two SCA FM subcarrier signals, the first being amplitude modulated and the second being phase modulated, to provide a digital data transmission system. A subcarrier signal within this system may also be modulated to carry audio signals.
A multichannel FM subcarrier broadcast system that provides a sequence of relatively closely spaced channels, using independent sidebands of suppressed carriers, is disclosed by Karr et al in U.S. Pat. No. 4,782,531. The sideband signals are generated in pairs and are phase shifted before transmission. Upon receipt of the transmitted signals, the process is reversed. An earlier patent, U.S. Pat. No. 3,518,376, issued to Caymen and Walker, discloses a similar approach without use of signal phase shifting of pairs of sideband signals.
In U.S. Pat. No. 4,799,062, Sanderford et al disclose a radio location method that uses a central processing station, a plurality of signal repeater base stations with fixed, known locations, and a mobile station with a known location at any time. The central station transmits a master grid synchronization pulse, which serves as a time reference, to the other stations at a selected sequence of times. A roving station with unknown location transmits a pulse that is received by three or more base stations and is retransmitted to the central station. The central station determines the location of the roving station using the differences in time of arrival at each base station of the pulse transmitted by the roving station. The mobile station (with known location) also transmits a pulse from time to time, and its known location is compared with its computed location by the central station to determine any multipath compensation required to reconcile the known and computed locations of the mobile station. The multipath compensation for a mobile station adjacent to the roving station is applied to correct the computed location of the roving station.
Ma, in U.S. Pat. No. 4,816,769, discloses receipt of SCA FM subcarrier signals for digital data paging at a radio receiver. The system measures signal-to-noise ratio of an output amplitude of a Costas loop, used to phase lock to the FM subcarrier frequency, to determine if the signal is sufficiently strong to be processed.
A system for detection of radio wave propagation time, disclosed by Ichiyoshi in U.S. Pat. No. 4,914,735, uses detection of phase differences for transmission of the signal over M (.gtoreq.2) different known signal paths to a target receiver. The transmitted signal includes a subcarrier signal, having a frequency that is higher than the transmitter clock frequency, modulated with a known modulation signal. The receiver has M demodulators for the signals received by the M different paths and has a phase comparator to compare the computed phases for each of these received signals. The phase differences are proportional to the signal path length differences, if compensation for transmission line distortions is included.
U.S. Pat. No. 5,023,934, issued to Wheeless, discloses a system for communication of graphic data using radio subcarrier frequencies. The data are broadcast on a subcarrier channel and received by a radio receiver that is connected to a computer. The computer receives the subcarrier signals, displays the graphic data on a computer screen, and performs other functions, such as transmission error checking and modification of the displayed graphic data. The system is intended for weather data communication and display.
Westfall, in U.S. Pat. No. 5,073,784, discloses a system for location of a transmitter ("unknown") at large distances, using a large network of pairs of spaced apart radio wave receivers whose locations are known and whose relative phases are synchronized. A signal, broadcast by the unknown transmitter at less than HF frequencies, is received at different time and space points by pairs of receivers. Simple geometrical computations allow determination of the location of the unknown transmitter by comparing times of arrival of the transmitted signal.
U.S. Pat. No. 5,170,487, issued to Peek, discloses use of FM sub-carrier signals for a pager system for mobile users. A plurality of transmitters are used, each of which transmits an FM subcarrier signal or a carrier signal modulated with a chosen message signal, slightly offset in time. Each page-receiving unit is assigned a time slot, during which the receiving unit dials through the set of frequencies corresponding to the FM subcarrier and modulated-carrier signals to determine if a page message has been sent for that mobile user.
A system that allows determination of an absolute location of a vehicle is disclosed by Kelley et al in U.S. Pat. No. 5,173,710. FM subcarrier signals are received from three radio stations with known locations but unknown relative phases by signal processors at the vehicle and at a fixed station with known location relative to the three radio stations. The fixed station processor determines the relative phases of the three radio stations FM subcarrier signals and broadcasts this relative phase information to the vehicle. The vehicle processor receives this relative phase data and determines its absolute location, using the phases of the FM signals it senses at its own location.
Chon, in U.S. Pat. No. 5,193,213, discloses an FM broadcast band system for receipt of relatively high frequency FM subcarrier signals. A tunable high pass receiver first circuit receives the carrier and a tunable low pass second circuit receives the subcarrier signal. Each signal can then be separately processed.
A navigation and tracking system using differential LORAN-C or differential Decca signalling is disclosed by Duffett-Smith in U.S. Pat. No. 5,045,861. A reference station transmits a reference signal to a mobile station and to three or more local LORAN-C or Decca (fixed) stations having known locations relative to the reference station. The fixed stations retransmit the reference signal to the mobile station, where the phase received signal differences are compared to determine the location of the mobile station.
Most of these systems use a single communication system, rather than integrating two or more communication systems to provide location or navigation information for a mobile user.The systems that use FM signals are limited to a region with a diameter of about 50 km or less. Any LD system that uses a plurality of radio carrier or subcarrier signals to determine location of a user must, at some point, determine the relative phases of these signals in order to estimate, by triangulation or related techniques, the user location. If the apparatus that determines these relative phases is fixed in location, this limits the flexibility and the. size of the region over which the LD system can be deployed. What is needed is apparatus: (1) that permits accurate estimation of the location of an object wherever the object is located on or near the Earth's surface and over regions of diameter hundreds or even thousands of km in diameter; (2) that is mobile or portable; (3) that can work indoors or outdoors; (4) that can provide estimates of location with inaccuracies no greater than ten meters, and more preferably no greater than one meter; and (5) that can determine the relative phases of radiowave carrier or subcarrier signals used as part of a hybrid LD system to determine the location of a mobile user. Preferably, the system should allow a choice between location information provided by two or more location determination systems, based on a comparison of one or more parameters that measure signal robustness and/or signal quality and/or system location for the signals received and analyzed by each communication system. Preferably, in one mode of operation the system should allow determination of location using three fixed signal sources and a mobile phase detection source.